Technological Field
The present invention relates to an image forming apparatus and a failure point estimation method.
Description of the Related Art
In electrophotographic image forming apparatuses, image abnormalities may occur due to the failure of parts relating to the electrophotographic process in some cases. For example, there are cases where a photosensitive drum which is a part relating to the developing process is damaged and generates streaks or a lack in the image. With respect to this, a test image is formed on the sheet, and read image data obtained by reading the test image is analyzed to detect image abnormalities. In order to solve the generation of image abnormalities, it is desirable to identify the failure point from the analysis result of the read image data.
As a method for identifying the failure point, there is a method of analyzing whether same image abnormalities periodically occur in the conveyance direction (sub-scanning direction) of the sheet. The electrophotographic image forming apparatus has many parts which convey the sheet or the image by rotating rotation bodies, and image abnormalities possibly occur at intervals of the rotation periods of the respective parts. For example, in a case where the image abnormalities occur due to the photosensitive drum being damaged, when the diameter of the photosensitive drum is 80 mm, the image abnormalities occur at intervals of approximately 251 mm. In this case, if the generation interval of the image abnormalities is found to be 251 mm from the analysis result of the read image data, it is possible to estimate that the failure point is the photosensitive drum.
Some of the parts which possibly cause the image abnormalities have periods which are longer than the length in the conveyance direction of the sheet size which is generally used. In a case of such a part, it is not possible to obtain read image data sufficient for analysis of the periods of the image abnormalities from only a single sheet. In a case of the image forming apparatus having the photosensitive drum with the outer circumference of 251 mm as described above, when the sheet of A3 size is used, the length in the conveyance direction of the sheet is 420 mm. Thus, the image abnormality of 251 mm period only appears at one or two points in one sheet. Accordingly, in order to raise the accuracy of period analysis, it is desirable to obtain as long data in the sheet conveyance direction as possible.
Thus, there is considered a method of performing analysis by continuously outputting a plurality of sheets on which the test image is formed and handling the read image data of the plurality of sheets as data which is continuous in the sheet conveyance direction. When handling the data which is continuous in the sheet conveyance direction, it is necessary to consider the distance (distance between sheets) from the rear end of one sheet to the front end of the next sheet. The theoretical value of the distance between sheets can be generally calculated from the printing speed for each sheet size.
For example, for an apparatus that continuously prints a test image on a plurality of sheets and that detects density irregularities in the sub-scanning direction from the measurement data obtained by optically reading the plurality of sheets, there is suggested a technique connecting the measurement data of each sheet with an interval corresponding to the distance between sheets (see JP 2011-87285 (A) and JP 2014-6336 (A)).
However, actually, the sheet is not necessarily conveyed at the distance between sheets as in the theoretical value due to the influence of variation in the sheet conveyance timing and the like. For example, when the sheets are handled on the sheet feeding tray and supplied one by one, the sheet may slip on the sheet feeding roller and the sheet conveyance timing may be shifted in some cases. In such a case, if the analysis is performed by assuming that the sheet is conveyed with the distance between sheets as in the theoretical value, it is not possible to accurately obtain the generation interval of the image abnormalities. Specifically, even when the image abnormalities actually occur at the period of 251 mm corresponding to the photosensitive drum, there is a possibility that the period cannot be accurately detected due to the gap of the theoretical value of the distance between sheets and the actual variation. There is also considered a method of providing a margin to the distance between sheets and comparing the interval between image abnormalities with the period corresponding to each part. However, the accuracy is lowered by the amount of the margin. In a case of providing the margin, though the range of variation needs to be grasped in advance, the range of variation is different also for the type of the sheet feeding unit. Thus, it is not easy to set an appropriate margin.